1. Field of the Invention
The present invention relates generally to telephone circuitry and more particularly to an attenuator device for attenuating the power level at telephone system interfaces.
2. Description of the Prior Art
For various reasons, telephone line circuits in different areas or different telephone systems operate at varying signal power levels. Therefore, it is often desirable to introduce signal power loss into a network to achieve a signal power balance between interfacing systems. This balance minimizes signal reflection. The process of introducing signal power loss into a network is known as attenuation and the devices for accomplishing attenuation are known as attenuators.
It is also desirable to attenuate signal power to minimize talker echo and "singing" in a telephone connection. Talker echo and singing in a telephone conversation are undesirable characteristics which detract from the subjective quality of the connection. It has been found that attenuating the signal power can reduce the detrimental affects. Empirical testing has produced standards for optimal attenuation based on the trade-off between signal power reduction and echo and singing.
Telephone engineers commonly utilize standard tables to determine the amount of signal power attenuation required in a given circuit. The signal power values are expressed in terms of dbm (decibels, absolute) for absolute power measurement as compared to a set base value or in db (decibels) for relative signal power measurement. These tables assume ideal conditions and are dependent on such factors as the mileage between end points of the circuit and the nature of the end points.
However, ideal conditions are rarely obtained. Line resistance and other intentional or unintentionally introduced transmission signal power losses typically reduce the power of the incoming signal by estimable but not precisely predictable amounts for any given point-to-point connection. The interposition of various elements within the circuit may also affect the power levels. Therefore, it is necessary to empirically select the amount of attenuation necessary for each interface.
Prior art attenuation devices were originally in the form of bulky resistance networks mounted on vacuum tube type bases which were plugged into the telephone line circuits at central telephone offices. It was not uncommon to see whole rooms full of these attenuation devices for a relatively small number of interfaces. Subsequently, improved technology led to plug-in attenuator devices of much smaller size. These were typically constructed using microtechnology such as thin and thick film conductor and resistor elements. An attenuator device of this nature is shown and described in U.S. Pat. No. 4,220,834 issued to Thomas J. Holce, et al.
A multiple component plug-in device is disclosed in U.S. Pat. No. 3,908,178, issued to B. E. Johnson and J. E. Danneman. This device uses fewer parts and is somewhat less bulky than other prior art attenuator devices but still requires a large number of components to effectively cover the range of attenuation necessary for telephone uses. The Johnson disclosure further contemplates a three component combination which has multiple connection points and inherent construction complexities which may lead to errors.
Other prior art attenuator devices, such as those including rheostats for adjusting the resistance of specific elements have also been utilized. Devices of this nature have typically been very large and have required significant volume for installation in addition to being unreliable.
The major factors creating disadvantages with prior art attenuation devices are space problems and the unpredictability of the precise attenuation required at a given interface. Those prior art devices which have permitted modification of the power attenuation at a given interface, such as those using rheostats, have been large and thus unacceptable from a spatial standpoint. On the other hand, the small plug-in devices such as disclosed by Holce et al are single attenuation value devices. That is, each plug-in device is permanently set to a given attenuation value. Telephone line systems frequently encounter attenuation requirements from 0.1 db all the way up to 32.5 db. If it is desired that this range be coverable in increments of 0.1 db, then it is necessary for the telephone company to maintain a stock of three-hundred-twenty-five (325) different plug-in attenuator devices.
When an installation is to be made at a given interface, the attenuation required can only be roughly estimated beforehand. Therefore, it frequently occurs that an installer will arrive at the interface point and determine that the plug-in device or devices which he has brought with him are not the ones required at that particular interface. Thus, it is frequently necessary to requisition new parts, make multiple trips to the installation site and otherwise expend unnecessary time, resources and energy in obtaining the proper attenuation. Furthermore, it is not uncommon for circuits to be reassigned or modified such that it becomes necessary to alter the amount of attenuation.